The state of the art provides a variety of machines for facing stone materials. Such machines make conventional use of diamond abrasive rollers in which the abrasive material extends in helical paths around a cylindrical surface of the roller to cut in the whole surface of a stone material being processed. While it is true that at any one time during the process the generatrix line of the cutter envelope is contacting the workpiece surface only with a section of its helix, still the cutting action will affect the whole workpiece area spanned by the generatrix line on account of the contact point moving in succession all along the helix as the roller is rotated. The net result is that the whole surface of the workpiece spanned by the generatrix line is processed at once. However, a facing machine equipped with such rollers is bound to apply a high pressure to the workpiece material, reflecting on increased power requirements and wear of the diamond abrasive. A limit is placed on the power used, and hence on the production output, by the frail nature of the workpiece material, since a belt type of transport cannot be provided that is totally immune to deformation and would not strain the material beyond its breaking point. This means that a controlled amount of power must be delivered to each abrasive roller.
Also known in the art is a calibrating machine for granite slabs having, located upstream of segmental grinding wheels, a pair of grooving rollers comprised of a set of disk cutters that are formed with radial teeth in order to face a slab surface and calibrate it to thickness by cutting grooves along the feed direction of the slab. The process is carried out on the back side of the slab, so that ridges can be left on this surface, if desired for later anchoring of the slab to a foundation in a more positive manner. Subsequently at a grinding station, the process is completed by a single diamond abrasive ring plate planarizing the slab, if necessary by smoothing away the ridges between grooves.
However, the performance of this calibrating machine has not proved much of an improvement on helical abrasive roller calibrating machines, mainly because the large number of disks set side by side on the grooving rollers are difficult to adjust for a sufficient number of narrow grooves and ridges to be produced. By reducing the number of disks, the slab surface could be processed more accurately but the ridges formed on the slab surface would be wider, thereby lowering significantly the working rate of the diamond abrasive ring plates.
In addition, the above prior technique, when applied to the face side of hard-fired ceramic tiles rather than the back side of granite slabs, involves frequent tool adjustment if depths are to be achieved between ridges with very close approximation, and prevents full use of the productive potential of modern vertical-axis rotary heads because differences in depth are liable to affect both the output and removing effectiveness of up-to-date rotary abrasive tooling for such heads.
Last, whereas in the instance of the above conventional calibrating machine with diamond abrasive ring plate grinders any machining inaccuracies would occur on the back side, away from view, for hard-fired ceramic tiles the machining process is directed to bring out a desired manufacturer's pattern or logo by removing a surface layer of perhaps a few tenths of a millimeter. This surface layer often leaves the kiln in a rippled state that is the outcome of previous tile molding steps as well as the baking step itself. Also, hard-fired ceramic tiles are smoothed to achieve a required degree of planarization for the subsequent polishing operation, so that their face side need be smoothed with the utmost accuracy.